(Radiology. 1999;210:227-232.)
© RSNA, 1999
Mammography Grid Performance
Paul S. Rezentes, PhD1,
Adelaide de Almeida, PhD2 and
Gary T. Barnes, PhD1
1 Department of Radiology, University of Alabama Hospital and Clinics, 619 S 19th St, Birmingham, AL 35233 (P.S.R., G.T.B.)
2 Faculty of Physical Sciences, Universidade de Sao Paulo, Campus de Ribeirao Preto, Brasil (A.d.A.).
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Abstract
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PURPOSE: To measure directly the grid performance of mammography units for the range of breast thicknesses and x-ray tube potentials encountered in clinical practice.
MATERIALS AND METHODS: Contrast improvement factors and Bucky factors were determined for four mammographic units as a function of x-ray tube potential (25, 30, and 35 kVp), phantom thickness (2, 4, and 8 cm) and, on one unit, three target-filter combinations. Three units used a linear grid; one, a cellular grid. Two methods were used for nongrid measurements.
RESULTS: For all units tested, contrast improvement factor increased with increased phantom thickness and with increased kilovolt peak level for the 8-cm-thick phantom and changed little with kilovolt peak level for 2- and 4-cm-thick phantoms. At 25 and 30 kVp, contrast improvement factor performance with the linear grids was comparable; with the cellular grid, it was 5%–10% higher. In all cases, the Bucky factor increased with increased phantom thickness and decreased with increased tube potential.
CONCLUSION: Differences in grid performance exist. At 25 and 30 kVp, the cellular grid exhibited superior contrast improvement factor performance, whereas one of the linear grids exhibited superior Bucky factor performance. Measured contrast improvement and Bucky factors are dependent on nongrid technique. Cassette tunnels introduce scatter and should not be used with nongrid or magnification techniques.
Index terms: Breast radiography, quality assurance, 00.11 • Breast radiography, technology, 00.11
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Introduction
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The deleterious effects of scatter on mammographic image contrast have been well documented (1–4). At present, the most widely used technique for scatter control is the use of a grid. In mammography, as in general radiography, use of a grid results in improved image contrast at the expense of increased dose to the patient. Although grids are widely used during mammography, few data about grid performance are available. Several investigators have used Monte Carlo simulations to compare grid performance and optimize grid design (4–8), whereas others have inferred performance from scatter-to-primary measurements with a single mammographic unit (9,10) or have evaluated grid performance qualitatively with clinical images, phantom images, or both (7,9,11–13). In the present study, we compared the grid performance of four mammographic units. Contrast improvement factors and Bucky factors were directly measured for a range of breast phantom thicknesses and x-ray tube potentials relevant to clinical practice.
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MATERIALS AND METHODS
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Grid performance was investigated with four mammographic units: the MAM-CP (Transworld, Charlotte, NC), Senographe 600T HF (GE Medical Systems, Milwaukee, Wis), DMR (GE Medical Systems), and M-III (Lorad, Danbury, Conn) units. Three of the units (MAM-CP, 600T, and DMR) use linear grids, whereas the M-III unit uses a cellular grid. Table 1 lists the manufacturer specifications for the grids studied. Nominal tube potentials of 25, 30, and 35 kVp were used with all except the M-III unit, with which the maximal tube potential of 34 kVp rather than 35 kVp was used. A molybdenum target–molybdenum filter (Mo-Mo) combination was used for all units; for the DMR unit, molybdenum target–rhodium filter (Mo-Rh) and rhodium target–rhodium filter (Rh-Rh) combinations were also used. The x-ray tube voltage calibration was checked (Triad System; Keithley Instruments, Cleveland, Ohio) for each unit and was found in all cases to be within 0.5 kV of the nominal value at 25 and 30 kVp and to be within 1.0 kV of the nominal value at 34 or 35 kVp. The measured half-value layer for the Mo-Mo combination of the four units (without the compression paddle) ranged from 0.30 to 0.33 mm of aluminum at 30 kVp.
The contrast improvement factor is a measure of the increase in contrast attained by using a grid and is equal to the ratio between the imaged subject contrast obtained with the grid and that obtained without the grid (5–10). The subject contrast SC is defined as SC = loge(
O/S), where O and S are the energy fluences absorbed in the intensifying screen associated with the object and with the surround, respectively (14). Subject contrast was measured with and without a grid on each unit by imaging a 0.25-mm-thick, 2 x 2-cm simulated calcification target placed centrally on a phantom with the equivalent of 50% glandular and 50% adipose tissue (CIRS, Norfolk, Va). The elemental composition of the target was 1.9% hydrogen, 25.4% carbon, 40.5% oxygen, 0.6% nitrogen, 30.7% calcium, and 1.4% bismuth. The mass attenuation coefficient of the target matched that of calcium carbonate to within 1%, 3.5%, and 6% at 20, 30, and 40 keV, respectively (Lundquist J, written communication, 1998). The 0.25-mm thickness was chosen to obtain a subject contrast in the range of 0.1–0.5 for the experimental conditions studied.
Measurements were performed for phantom thicknesses of 2, 4, and 8 cm. The phantom dimensions were 12.4 x 12.4 cm. The optical density was measured in the center of the target and immediately adjacent to and on the left and right sides of the target. The left and right adjacent measurements were averaged. The difference in optical density between the target image and the adjacent area was converted to the subject contrast by using film sensitometry (Processor Control Sensitometer; Eastman Kodak, Rochester, NY). The exposure time was adjusted to maintain optical densities in the range of 1.2–2.2 (ie, in the linear range of sensitometric response). The contrast improvement factor was determined for each combination of kilovolt peak level and phantom thickness, with the exception of 2 cm at 35 kVp, which presented experimental difficulties and lacked clinical relevance. For each contrast improvement factor value, six or more measurements of subject contrast were performed, and the results were averaged before the contrast improvement factor ratio was determined. The experiment made use of Detail S intensifying screens (Agfa, Ridgefield Park, NJ) in conjunction with MRE film (Eastman Kodak) and Spectrum chemistry (Picker International, Cleveland, Ohio) in an Agfa Curix Compact processor set at 35°C (95°F) and 120 seconds.
The Bucky factor is a measure of the increase in dose to the patient that results when a grid is used. Bucky factors were determined for each unit from the ratio of the exposure time (in milliampere seconds) needed to image the phantom (without the calcification target) with the grid to the exposure time needed without the grid to a net optical density of 1.25. The phantom usually was imaged at a net optical density of slightly less than and greater than 1.25, and time-scale sensitometry was used to estimate the exposure time associated with 1.25 net optical density. As in the case of contrast improvement factors, Bucky factors were determined for each combination of kilovolt peak measurement and phantom thickness, with the exception of 2 cm at 35 kVp and of 8 cm at 25 kVp for the M-III because of experimental difficulties. For each Bucky factor datum, three or more measurements were performed, and the results were averaged.
Two methods were used for measurement of the contrast improvement factor and Bucky factor nongrid parameters. With one method, the phantom was imaged while it rested directly on the cassette with the aid of a cassette holder (method 1); the other method involved use of a cassette "tunnel," which is equivalent to a standard Bucky assembly with the grid removed (method 2). The fundamental difference between the two methods was the use of a carbon fiber cover with the latter method. The nongrid measurements for the MAM-CP and M-III units were performed with method 1, whereas method 2 was used with the DMR unit (Mo-Rh and Rh-Rh combinations). For comparison, contrast improvement factors and Bucky factors were measured with both methods on the 600T and DMR (Mo-Mo combination) units.
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RESULTS
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Contrast improvement factors measured with methods 1 and 2 are presented in Tables 2 and 3, respectively. Bucky factors measured with methods 1 and 2 are presented in Tables 4 and 5, respectively. For all mammographic units, the general trends were that the contrast improvement factor increased with increased phantom thickness, increased with increased kilovolt peak and the 8-cm thickness in method 1, and changed little with kilovolt peak and 2- and 4-cm-thick phantoms. The Bucky factor increased with increased phantom thickness and decreased with increased kilovolt peak. These trends are shown in Figure 1 for the MAM-CP unit.
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Figure 1. Graph shows MAM-CP grid contrast improvement factor and Bucky factor versus kilovolt peak level for 2-, 4-, and 8-cm breast phantom thicknesses.
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The differences in results between the two methods are noteworthy. In Figure 2, the contrast improvement factors (Fig 2a) and the Bucky factors (Fig 2b) measured with both methods by using 4- and 8-cm thick phantoms on the 600T unit are plotted against x-ray tube potential. The values of the contrast improvement factor and Bucky factor are dependent on the method of measurement of the nongrid parameter. Contrast improvement factors are higher (5%–20%) and Bucky factors are lower (10%–20%) with method 2 than with method 1.
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Figure 2a. Graphs show 600T grid (a) contrast improvement factor and (b) Bucky factor versus kilovolt peak level for 4- and 8-cm-thick phantoms. These graphs illustrate the effect of the nongrid technique on measured grid performance. In method 1, the nongrid phantom image is obtained by positioning the phantom directly on the screen-film cassette; in method 2, the phantom is positioned on the cassette tunnel.
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Figure 2b. Graphs show 600T grid (a) contrast improvement factor and (b) Bucky factor versus kilovolt peak level for 4- and 8-cm-thick phantoms. These graphs illustrate the effect of the nongrid technique on measured grid performance. In method 1, the nongrid phantom image is obtained by positioning the phantom directly on the screen-film cassette; in method 2, the phantom is positioned on the cassette tunnel.
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In a comparison of measurements achieved with method 1, three units (MAM-CP, 600T, and DMR) had similar contrast improvement factors at 25 and 30 kVp, and the M-III unit demonstrated systematically higher contrast improvement factors. At 35 kVp (34 kVp for the M-III unit), the contrast improvement factor achieved with the MAM-CP unit for the 8-cm-thick phantom was slightly higher than the values achieved with the other three units. The Bucky factors for the 600T, DMR, and M-III units were similar and higher than the Bucky factors for the MAM-CP unit at 25 and 30 kVp. At 35 kVp, the Bucky factor for the MAM-CP unit was less with the 4-cm-thick phantom than the values measured on the other three units and slightly less with the 8-cm-thick phantom. With regard to the different target-filter combinations (method 2), for 25 and 30 kVp, a small systematic increase in the contrast improvement factor was evident for a given phantom thickness as we changed from the Mo-Mo to the Mo-Rh combination and from the Mo-Rh to the Rh-Rh combination. Although the difference between two adjacent data points (eg, Mo-Mo and Mo-Rh at 30 kVp and a 4-cm-thick phantom) was comparable to the uncertainty in the measurements, the trend is evident across the data set.
The average SD for the subject contrast measurements was approximately 3%, which corresponds to a contrast improvement factor measurement uncertainty of 4%–5%. The average SD of the Bucky factor measurements was approximately 3%. To minimize systematic errors in the measurement of the contrast improvement factors, the same sensitometer was used in all cases. Because the contrast improvement factors were determined on the basis of the ratios of subject contrast measurements, it is anticipated that the presence of a systematic error in the subject contrast measurements would be minimized due to cancellation. In clinical practice and in our Bucky factor measurements, failure of the film reciprocity law is an inherent contributing factor that may be dependent on the screen-film system used. Scientific data about failure of the reciprocity law regarding mammographic film are limited. The general trends, however, should be similar. With regard to the factor of the two- to fourfold increase in exposure time with the grid technique in comparison with the nongrid technique, 3%–15% of the increase and of the associated Bucky factors can be attributed to film reciprocity failure (15).
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DISCUSSION
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The observed general trends can be understood as follows. The increase in the contrast improvement factor with increasing phantom thickness is due to the corresponding increase in scatter emerging from the phantom and the potential for more contrast improvement (3). In a similar manner, the increase in the Bucky factor with increasing phantom thickness is related to the fact that, with more scatter, a greater fraction of the x-ray energy fluence that emerges from the phantom is absorbed by the grid, which results in an increased Bucky factor. These trends are consistent with the clinical findings of Sickles and Weber (16).
The higher contrast improvement factors measured at 25 and 30 kVp for the M-III unit and grid are attributed to the smaller scatter-acceptance solid angle associated with its cellular structure and its greater metal content (Table 1). Of practical importance is that this improved performance is achieved with a Bucky factor slightly less than the Bucky factors of the 600T and DMR units. In view of the fact that the Mo-Mo combination spectra of the four units were closely matched, the lower Bucky factor of the MAM-CP unit as compared with the other three units for all kilovoltages and phantom thicknesses is attributed to differences in grid construction between the grid in the MAM-CP unit and the other grids (Table 1). This grid has a higher strip density and a higher fraction of area occupied by lead than does the Smit-Röntgen grid but is thinner. Although the contrast improvement factor of the grid in the M-III unit was superior to that of the other grids at 25 and 30 kVp, its performance at 34 kVp with an 8-cm-thick phantom was comparable and is attributed to increased scatter penetration of its copper septa. The grids of the other units had lead septa.
In clinical practice, the majority of screen-film mammograms are obtained at x-ray tube potentials of 25–30 kVp. On occasion, however, for thick, dense breasts, tube potentials greater than 30 kVp are used (17). It is anticipated that with digital mammography, tube potentials will be higher than those in current use in screen-film mammography (14). For these reasons, measurements were also performed at 35 kVp.
In the present study, a calcification contrast target was used to determine grid contrast improvement factors. The calcification target was chosen because of its clinical relevance. Contrast improvement factors are not sensitive to the choice of target material, however, because the contrast improvement factor is the ratio of the target subject contrast obtained with the grid to the contrast obtained without the grid. We have performed contrast improvement factor measurements with aluminum targets and have obtained similar results. The measurement can be sensitive to target contrast. If the target contrast is too great, the scatter field is perturbed, which in turn will affect the resultant contrast improvement factor. The choice of target contrast is a trade-off between measurement accuracy and perturbation of the scatter field. If the target subject contrast is large, it is easy to measure but perturbs the scatter field. If it is small, it has little effect on the scatter field but is difficult to measure. In the present study, the thickness of the calcification target was chosen with this trade-off in mind for the range of scatter conditions studied. An alternative and probably superior method would be to vary the target thickness with the phantom thickness so that approximately the same subject contrast is obtained for the different phantom thicknesses studied.
Earlier grid performance results obtained by Nielsen and Fagerberg (9) and Wagner (10) were inferred from scatter-to-primary measurements and grid primary-transmission measurements. Nielsen and Fagerberg used a 4-cm-thick Lucite phantom at 25 kVp and a moving Smit-Röntgen grid similar to the one used in the present study. Their contrast improvement factor value (1.27) was comparable to the values we obtained with method 1. Their Bucky factor result, however, was considerably higher (3.23 vs 2.41) and is attributed to the difference between the methods used. Wagner's measurements were made with a Transworld unit with grid specifications identical to the Transworld unit in the present study (Table 1). Measurements were made at 25, 30, and 35 kVp with 2-, 4-, and 6-cm phantom thicknesses. The composition and area of the phantom and the half-value layer of the unit at 30 kVp were similar to those of the present study. For the common phantom thicknesses (2 and 4 cm), Wagner's contrast improvement factor values were systematically higher (approximately 10%), and the Bucky factor values were slightly lower (approximately 3%). Wagner's results were inferred from beam-stop scatter-to-primary measurements that were extrapolated to zero area. Such an approach includes small angle scatter and increases the measured level of scatter, which gives rise to higher contrast improvement factors than the direct measurement technique we used.
An important finding of our study is that the contrast improvement factor and Bucky factor in mammography are dependent on the method of measurement. These parameters are dependent on whether the nongrid measurements are performed without (method 1) or with (method 2) a cassette tunnel. With method 2, the carbon-fiber cover of the cassette tunnel attenuates the beam, increases the scatter that is imaged, and lowers target contrast in the nongrid measurement. Thus, more improvement in contrast is possible when a grid is used. In a similar manner, the Bucky factor is less with method 2. This finding has important implications as regards clinical practice. Nongrid images in patients will have higher contrast with the breast positioned directly on the cassette or, if a magnification technique is used, when the cassette is supported without the use of a cassette tunnel with a carbon-fiber attenuator directly above the cassette. At present, a number of mammography units use a cassette tunnel for nongrid techniques and, even worse, for magnification techniques. Our results suggest that this practice should be changed, and these results have been communicated to equipment manufacturers. In the future, all units should be supplied with a cassette holder, rather than a cassette tunnel, and magnification stands should be designed to eliminate the use of a tunnel. Furthermore, in our opinion, the manufacturers of units that were originally equipped with cassette tunnels should offer their customers cassette holders and improved magnification stands.
Differences in mammography grid performance exist: For 25 and 30 kVp, the M-III cellular grid (Lorad) exhibited superior contrast improvement factor performance, whereas the Takeuchi linear grid on the MAM-CP unit exhibited superior Bucky factor performance. Measured contrast improvement factor and Bucky factor values are dependent on the nongrid technique. Cassette tunnels introduce scatter and should not be used with nongrid or magnification techniques.
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Footnotes
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A.d.A. supported in part by grants from FAPESP (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo, Brazil).
Address reprint requests to G.T.B.
Abbreviations: Mo-Mo = molybdenum target–molybdenum filter
Mo-Rh = molybdenum target–rhodium filter
Rh-Rh = rhodium target–rhodium filter
Author contributions: Guarantors of integrity of entire study, P.S.R., G.T.B.; study concepts and design, P.S.R., A.d.A. G.T.B.; definition of intellectual content, P.S.R., G.T.B.; literature research, P.S.R., G.T.B.; experimental studies, P.S.R., A.d.A. G.T.B.; data acquisition and analysis, P.S.R., A.d.A.; manuscript preparation, P.S.R.; manuscript editing and review, G.T.B.
Received November 5, 1997;
revision requested February 13, 1998; revision received May 11, 1998;
accepted July 13, 1998.
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Abstract
Introduction
